Device for both-way transposition between optical signals and electrical signals, for a communications system

ABSTRACT

The present invention relates to a device for both-way transposition between optical signals and electrical signals, for a communications system, said device comprising a vertically integrated component comprising an optically driven oscillator based on a negative differential conductance mechanism, for performing optical-to-millimetric conversion on the down path, and an asymmetric Fabry-Perot cavity modulator for performing the electrical-to-optical conversion function on the up path.

The present invention relates to communications systems using opticalsignals and electrical signals.

More precisely, the present invention relates to a device for both-waytransposition between optical signals and electrical signals.

BACKGROUND OF THE INVENTION

Various fields of application require components making it possible toconvert optical signals (in general conveyed by optical fibers) intoelectromagnetic waves that can propagate through empty space. Thisapplies particularly to the field of telecommunications, in which, toconnect subscribers to specific services, it is preferable, for reasonsrelated to the system or to the service to be provided, to use radiowaves for the last leg from a terminal connected to the optical fiberdistribution network. Such a technique will be beneficial forgeneralizing portable subscriber terminals by fitting flexibly andcheaply into existing radiocommunications infrastructures.

In the field of telecommunications, demand is becoming increasinglyhigh-data rate oriented, whether it be for optical fiber distributionnetworks or for radio networks for mobile terminals. Essentially fortechnical reasons to do with passband, it would seem appropriate forthose two types of network to converge, with an optical fiber highdata-rate fixed network being extended through empty space by a radiobroadband access network operating at a very high frequency, i.e.typically a few tens of GHz (e.g. see references 1, 2, 3, 4, 5, 6, 7!).

In such a radio broadband access network at the ends of optical fibers,the radio coverage is provided by base stations distributed outside orinside buildings. Each of the base stations is connected via opticalfibers to an exchange. In particular, it has been demonstrated that, forthe purposes of serving buildings, such architectures are much moreadvantageous than networks having combined fiber-and-coaxial cable linksor even fiber links to the subscriber Ref. 6, 7!. Such high data-rateradio links grafted onto an optical fiber distribution network not onlyoffer all of the advantages that are related to mobility, but alsoenable major savings to be made on terminal wiring.

In architectures currently being researched Ref. 3, 4, 5!, the radiosignal, generated in an exchange, is conveyed in optical form by theoptical fiber distribution network towards the stations of the accessnetwork, in which stations optical-to-radio conversion is performed toprovide the link to the subscribers. Such an exchange-to-subscriber pathis generally termed the "down" path.

The design of the opposite, subscriber-to-exchange, or "up" path is morecomplex. The information arriving at each station of the access networkmust be converted into optical form. This is the same function as isperformed on the down path, where it is centralized for reasons ofeconomy, but in general up-path conversion takes place at data ratesthat are lower.

A difficulty encountered by attempts to develop such future networks isthe problem of providing reliable and cheap active components for radioterminals.

A certain number of components have already been developed, making itpossible to perform the optical wave-to-millimetric wave conversionfunction separately for the down path from the exchange and for the uppath returning from the station. Such a component is constituted by ahybrid optical-millimetric duplexer transposer incorporating unitcomponents of the following types in the same module: detector,oscillator, coupler, light source, and modulator. Assembling such acomplex set of components has a considerable impact on the overall costof the terminal.

In a radio-over-fiber link of the type described in the above-mentioneddocuments, the exchange is connected by optical fiber to a certainnumber of stations each of which is equipped with an antenna.

For the down path, the radio signal is applied to the optical carrier atthe exchange.

Two approaches are under consideration, depending on whether or not thesignal includes the radio carrier. At the station, the radio signalextracted from the optical carrier feeds the antenna which communicatesby radio through empty space with the mobile terminals. If it exists onthe optical carrier, the radio carrier comes directly fromoptical-to-radio conversion. Otherwise, it is generated by a localoscillator.

For the up path, the radio signals picked up by the antenna of thestation modulate an optical carrier generated by a light source. Theresulting optical wave is then taken to the exchange over a fiber thatis different from the fiber used for the down path, or possibly evenover the same fiber.

At the receiver end of the down path, at the station at the end of theoptical fiber, the function to be performed is optical-to-millimetricconversion.

It can be performed merely by photodetecting the optical signal comingfrom the optical fiber, either in an ultra-fast photodiode followed bytransistor amplification 8! or directly in a phototransistor, whichmakes it possible in addition to provide gain in a more integratedmanner 9!. Implementing that solution has shown that the radio signalpower extracted from the optical carrier remains rather low in spite ofthe phototransistor. To provide a power level that is high enough forapplication to the antenna, a microwave amplifier must be added which iscomplex and costly, especially at high frequencies, such as thoseplanned in such systems.

To mitigate that limitation, consideration is currently being given toan alternative approach that is more advantageous as regards power. Itconsists in using an optically controlled millimetric oscillator. Itwould appear to be cheaper provided that sufficiently powerfuloscillators can be implemented cheaply, e.g. with unitary componentsusing technology that is much easier than transistors for millimetricamplifiers. Several works have been published on millimetric sourcescontrolled by optical signals 10, 11, 12, 13!. More particularly, theprinciple of using the high current gain of a 1.3 -μm light-sensitivephototransistor by integrating it in an oscillator circuit having highoutput power and possessing a wide locking range at low incident opticalpower has been demonstrated 14!. Unfortunately, the frequencies obtainedin that way remain rather low for the moment.

The above-described approaches handle optical functions separately fromelectrical functions. An original solution associating both types offunction in a common component has been proposed and recently testedsuccessfully in a system experiment 16!. It consists in using asuperlattice millimetric diode having negative differential conductance15, 16!.

At the transmitter for the up path, in the end station at the end of theoptical fiber, the function to be performed is an electrical-to-opticalconversion. On that path, the electrical signal is assumed to resultfrom demodulation, and it is therefore in base band. That solution,currently being experimented at system level, uses a semiconductor laser16!. The laser is modulated directly by the electrical signal comingfrom the antenna, and it returns the information to the exchange at awavelength identical to or different from that of the down path,depending on the chosen coding. The structures are nevertheless quitecomplex and are implemented using light-guide technology which posesassembly and cost problems.

The major drawback of the above-mentioned state-of-the-art system is itscomplexity related to the number of sophisticated components itrequires, with obvious consequences on its overall cost. A point to beemphasized more particularly is that the functions are totally separatedon the two paths: each function is performed by a component which mustbe both electrically and optically inserted in the flow of information.The costs of assembly and interconnection (in particular for opticaltechnology) form a large portion of the overall cost of the system,especially since the components usually implemented (lasers ofmodulators) operate under guided propagation conditions.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to simplify a both-way optical-millimetriclink so as to reduce its cost. To achieve this object, it is necessaryto reduce the number of sophisticated components used, thereby reducingthe number of interconnections (in particular optical interconnections).Such interconnections must also be facilitated by means of afiber-drawing technique that does not require very high precision.Similarly, the invention aims to choose appropriate technologies tolimit the precision constraints that are often imposed whenmanufacturing high-speed components.

In some ways, these concerns tie in with those that are guiding researchinto components for optical terminals in the context of bringing thefiber to the home (FTTH). Such research has already led to a certainnumber of implementations that can, in principle, be adapted by a personskilled in the art to suit the optical-radio field. Examining them showsthat, even if such adaptation is successful, a certain number ofdrawbacks will be encountered, to which the invention proposes toprovide solutions.

A first line of research currently being followed actively for theoptical access network consists in omitting the light source at thesubscriber end. In which case, the terminal is constituted by anopto-electronic modulator operating as a mirror 17, 18!. The mirrorreceives a light wave emitted from the exchange at a wavelengthidentical to or different from that of the down path, and returns it tothe exchange after applying to it the electrical signal delivered by thesubscriber. That solution may be termed "passive". Three types ofcomponent have been developed to perform that function. Firstly,in-waveguide Mach-Zehnder type interferometers have been made of lithiumniobate and tested experimentally in various system configurations 19!.Secondly, in recent years, electro-absorbant modulators have beendeveloped that have vertical structures and that can be subjected tofiber-drawing more easily than previous modulators 20, 21, 22!. Thosecomponents are III-V semiconductor structures of the surface-accessasymmetric Fabry-Perot cavity type formed by epitaxy on GaAs or InPsubstrates. Thirdly, structures of the hollow Fabry-Perot cavity typewith an air gap between the two mirrors have been tested experimentallyin system configurations in which the data-rate for the up path is quitelow 23, 24!.

Using such components for radio-optical transposition would offeralternative solutions for the up path but it does not deal with themajor concern of the cost of physically uniting the two paths byimplementing a single component.

A second approach, also actively under investigation for opticaldistribution purposes, uses two duplexers capable of performingall-optical transmission functions and reception functionssimultaneously.

In that approach, there are two classes of component.

The first class associates a photodetector and a filter isolating theinput from the output in a common module in hybrid manner, byimplementing a light-guide structure on a common substrate 25!. A devicethat is more integrated and that offers the above possibility hasrecently been developed 26!. That device, which is cheaper than thepreceding device, integrates a laser and a photodetector that are fullydecoupled on a common substrate and in a common waveguide connected to asingle fiber. It uses multi-segment DFB laser type structures whosetechnologies are now well developed.

The other class of device, which is more advantageous as regardsfiber-drawing, associates a photodiode and a Fabry-Perot mirror of thesame type as described above in a common vertical structure obtained ina single epitaxial growth step 27!. That type of component leads tocentralized architecture. In spite of that, like the former class ofcomponents, the latter class, when extended to the optical-millimetricfield, suffers from drawbacks already emphasized. In particular, sincethey use a photodiode for the photodetection function, they remain verylimited as regards the power level of the output microwave.

To mitigate the drawbacks of the devices proposed in the state of theart, the present invention provides a vertically integrated componentcomprising an optically driven oscillator based on a negativedifferential conductance mechanism, such as a Gunn diode, an IMPATTdiode, or a superlattice diode, for performing optical-to-millimetricconversion on the down path, and an asymmetric Fabry-Perot cavitymodulator for performing the electrical-to-optical conversion functionon the up path.

The term "optically driven" applies to two situations, namely a"locking" situation and a "mixing" situation, both of which can beenvisaged in the context of the present invention, in two distinctsystem architectures:

either the radio carrier is generated by the oscillator itself, in whichcase the oscillator acts as a mixer for mixing its characteristicoscillation, imposed by the characteristics of the circuit in which itis inserted, with the data-carrying signal that arrives at it optically;

or else the radio carrier is conveyed by the optical distributionnetwork, and the frequency of the oscillator is servo-controlled andsynchronized continuously on the instantaneous value of the millimetricfrequency present on the light wave, in which case the oscillator issaid to be "optically locked". Such optical locking is direct if theoptical signal servo-controls the oscillator itself, and it is indirectif the oscillator is locked by an electrical signal output by thedistinct photodetector.

The device of the present invention operates alternately orsimultaneously, for the down path, as an optically driven oscillatorcapable of delivering sufficient microwave power, and, for the up path,as a mirror modulator driven by a radio signal received by an antenna,thereby modulating an optical carrier sent by an exchange. The down pathand the up path can be conveyed over two fibers, or, more cheaply, overa common fiber.

The present invention thus makes it possible to reduce the number ofcomponents necessary for the both-way opto-radio link function.

Furthermore, the device of the present invention can be easily subjectedto fiber-drawing which constitutes a considerable advantage as regardslowering the cost of the end device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics, objects, and advantages of the present inventionappear on reading the following detailed description with reference tothe accompanying drawings which are given by way of non-limitingexample, and in which:

FIG. 1 is a diagrammatic vertical section view through a firstembodiment of a component of the invention;

FIG. 2 shows the reflectivity spectrum of the integrated mirrormodulator and superlattice oscillator structure of FIG. 1;

FIG. 3a shows a simulation of the internal potential in an integratedmirror modulator and superlattice structure of FIG. 1;

FIG. 3b shows the time response consequent upon exciting thesuperlattice oscillator only, with a short (1 ps) optical pulse (thedensity of the light-generated carriers is about 10¹⁵ cm⁻³);

FIG. 4 is a diagrammatic vertical section view through a variant of anintegrated mirror modulator and superlattice oscillator structure havingfour contacts;

FIG. 5 is a diagrammatic vertical section view through an integratedhollow-cavity Fabry-Perot mirror and superlattice oscillator structure;

FIG. 6 is a plan view of the hollow-cavity Fabry-Perot mirror portion;and

FIG. 7 shows the reflectivity spectrum of the integrated hollow-cavityFabry-Perot mirror and superlattice oscillator structure of FIG. 5.

MORE DETAILED DESCRIPTION

Two embodiments of devices of the present invention for both-waytransposition between optical signals and electrical signals aredescribed below.

First embodiment

The first embodiment is constituted by an integrated vertical structurecomprising a superlattice oscillator 20 and a Fabry-Perot mirrormodulator 30.

FIG. 1 diagrammatically shows this structure in which an oscillatorsegment 20 and a modulator mirror segment 30 superposed thereon areintegrated vertically by being grown on a common substrate 10.

The component shown in FIG. 1 is provided with three electrodes 26, 27,36 including an electrode 27 that is common to both of the segments 20and 30. It is explained below with reference to FIG. 4 that, in avariant, the component may include four electrodes 26, 27, 36, 37, toprovide better decoupling between the two segments 20 and 30.

More precisely, the superlattice oscillator 20 comprises the followingdeposited in succession on the semi-insulating substrate 10;

an n-type contact layer 21;

a graded layer 22;

a superlattice 23;

a graded layer 24; and

an n-type contact layer 25.

As shown in FIG. 1, a first metal electrode 26 is deposited on the firstcontact layer 21 while a second metal electrode 27 is deposited on thesecond contact layer 25. For this purpose, the first contact layer 21extends beyond the layers 22 to 25 that are superposed on it so as tohave an uncovered surface portion accessible to the electrode 26.

The Fabry-Perot mirror modulator 30 comprises the following deposited insuccession on the contact layer 25:

an n-type Bragg mirror 31;

a multiple quantum well layer 32; and

a p-type Bragg mirror 33.

The stack of layers 31, 32, 33 is narrower than the contact layer 25 soas to leave uncovered a surface portion thereof accessible to theelectrode 27.

An annular metal electrode 36 is deposited on the Bragg mirror 33.

From the optical point of view, the incident light λ1, λ2 coming from anexchange is injected via an optical fiber 40 into the top surface of theBragg mirror 33 at the center of the electrode 36, and the light λ2reflected by the modulator 30 is collected in the same fiber 40. Theoperating wavelengths for which the structure is designed are chosen asa function of the architecture of the system. In a configuration inwhich the link operates in full-duplex manner, the optical fiber 40injects at least two wavelengths λ1 and λ2 into the top surface of themodulator 30. One of the wavelengths, e.g. λ2, is allocated to the uppath, the other λ1 being allocated to the down path. The modulatormirror 30 is active at λ2 and transparent at λ1, while the oscillator islight-sensitive at λ1. To avoid crosstalk between the two segments, itis preferable to have λ1 >λ2.

Operation of the modulator mirror 30 is based on the electro-absorbanteffect at λ2, i.e. the variations in the optical absorption at thiswavelength induced by the variations in the voltage applied to thisportion of the structure. In the active zone 32, semiconductor materialsare used whose optical absorption edge wavelength is in the vicinity ofthe working wavelength of the modulator. The effect is even morepronounced if quantum wells are used in the active zone.

The modulator 30 thus forms a multiple quantum well asymmetricFabry-Perot resonator operating in reflection mode. The cavity isdesigned firstly to have a stop band centered on λ2 and a Fabry-Perotpeak in the vicinity of λ2, and secondly to be transparent to λ1.

Since the back mirror 31 must have high reflectivity, it is constitutedby a periodically alternating succession of materials of high refractiveindex and of low refractive index, and of quarter-wave thickness (Braggmirror).

The front mirror 33 may also be a Bragg mirror of lower reflectivity ormerely be constituted by the semiconductor-air interface.

The number of periods of the Bragg mirrors 31 and 33 are set so that thereflectivity is almost zero at λ2.

Under the effect of a voltage applied between the two mirrors 31, 33 viathe electrodes 26 and 27, the absorption and the refractive index aremodified in the cavity. This results in a spectrum shift and a change inthe intensity of the Fabry-Perot peak of the cavity and thus of thereflectivity of the structure at the working wavelength (λ2). Thereflectivity contrast between two bias states depends to a large extenton the quality of the near-extinction that it is possible to obtain inone of the states. These effects are well known, as is the design of thesuccession of materials required to obtain them.

Depending on the types of material making up the top mirror 33 when itis a Bragg mirror, there are two variants. In one of the structures, themirror 33 may be made up of insulating dielectrics (e.g. an alternatingsuccession of SiO₂ and TiO₂) deposited using a conventional method(evaporation, sputtering, etc.). In this case, the deposition for theelectrical contact 36 is performed prior to depositing the mirror 33. Inthe other structure, the mirror 33 is formed of a stack of semiconductormaterials with ad-hoc doping.

The oscillator 20 that is light-sensitive at λ1 is constituted by asuperlattice 23 that is light-absorbant at λ1. Operation of it is basedon the "negative differential velocity" related to the dynamics of thecharge carriers within the miniband resulting from the superlatticeeffect, and depends on the microwave or millimetric-wave circuit inwhich it is inserted (in particular on the Q factor of the circuit). Bymodifying the charge density in the miniband by optical injection, theoscillations are locked effectively, the frequency of the oscillationsthen being tied to that of the radio subcarrier of the optical carrier.It is also possible to implement local oscillator and mixer typeoperation when the radio carrier is not applied to the optical carrier.The oscillator then mixes its characteristic frequency with thefrequency of the radio signal applied to the optical carrier. With thistype of component, good optical-electrical conversion efficiency isobtained leading to comfortably high radio power for applying to theantenna. The frequency of the oscillator 20 is determined on the basisof the parameters of the superlattice 23 and by means of the design of atuning circuit using well-known engineering rules.

The structure is of the multilayer type. It is essentially constitutedby a succession of different III-V alloys.

The entire structure is grown in a single epitaxy step on asemiconductor substrate. The component is then processed by using theusual III-V semiconductor technology methods: lithography, etching,deposition of electrical contacts, etc.

In a non-limiting example, the device of the present invention comprisesthe following succession of layers grown epitaxially on amatched-lattice InP substrate 10 for operating in the vicinities of thewavelengths 1.55 μm and 1.3 μm:

A first set 21-25 of layers forms the oscillator 20 that islight-sensitive in the vicinity of 1.5 μm:

1) An n-type (10¹⁹ cm⁻³) GaInAs contact layer 21 having a thickness ofabout 7,000 Å.

2) A superlattice 23 made of InGaAs/InGaAlAs or some other material thatis active in the vicinity of 1.5 μm, n-type doped at about 2×10¹⁶ cm⁻³.By way of example, the following pair of materials may be chosen:InGaAs/(InGaAs)₀.5 (InAlAs)₀.5, with wells of 60 Å, barriers of 20 Å,and a total thickness of 9,165 Å, for operation in the vicinity of 50GHz.

3) A contact layer 25 made of GaInAs of the n-type (10¹⁹ cm⁻³) having athickness of about 2,000 Å. The exact thicknesses of the layers 21, 23,and 25 may be chosen so that they are mλ/2 thick (where m is aninteger), λ being the reference wavelength for the design of themodulator 30. The layer 25 must absorb the residual 1.3 μm which has notbeen absorbed or reflected in the modulator 30, so as to isolate theoscillator 20 optically from the modulator 30. Its thickness may be1,845 Å.

A second set 31-33 of layers forms the 1.3 -μm modulator 30:

4) A Bragg mirror 31 reflective in a band centered on 1.3 μm, or even adual mirror reflective in two bands centered on 1.52 μm and 1.3 μm,constituted by an alternating succession of layers that are n-doped atabout 10¹⁸ cm⁻³, and that are mostly λ/4 at λ=1.414 μm, e.g. 5 periodsof an alternating succession of 10 layers as follows:

1,034 Å of (InGaAs)₀.62 (InAlAs)₀.38

1,099 Å of InAlAs

1,034 Å of (InGaAs)₀.62 (InAlAs)₀.38

1,099 Å of InAlAs

1,034 Å of (InGaAs)₀.62 (InAlAs)₀.38

2,198 Å of InAlAs

1,034 Å of (InGaAs)₀.62 (InAlAs)₀.38

1,099 Å of InAlAs

1,034 Å of (InGaAs)₀.62 (InAlAs)₀.38

1,099 Å of InAlAs

5) A non-doped active layer 32 absorbant at 1.3 μm and constituted by11,014 Å of (InGaAs)₀.73 (InAlAs)₀.27. A portion of this layer may bep-doped (5×10¹⁸ cm⁻³) to reduce the thickness of the non-doped zone.

6) A p-doped Bragg mirror 33 (5×10¹⁸ cm⁻³) constituted by 2 layers asfollows:

1,005 Å of InAlAs

938 Å of (InGaAs)₀.62 (InAlAs)₀.38

Over a small portion of its thickness, each of the contact layers 21 and25 has a gradually changing composition 22, 24 serving to eliminatesudden discontinuities in the conduction band.

In this example, the metal electrode 27 deposited on the layer 25 iscommon both to the oscillator segment 20 and to the modulator segment30.

A model of the reflectivity of the entire set of layers over the range1.2 μm to 1.6 μm is shown in FIG. 2. This model shows that thereflectivity is almost zero at the wavelengths λ1=1.52 μm and λ2=1.3 μm.The electro-absorbant effect takes place in the vicinity of λ1, itsinfluence is small on the Fabry-Perot peak in the vicinity of λ2 becausethis wavelength is distant from the absorbent region and it is subjectedonly to small variations in index.

FIG. 3 shows the result of a two-dimensional electrical simulation for amultilayer structure similar to the above-described structure from theelectrical point of view, but with a simplified stack of layers for theBragg mirror 31 (only three periods). FIG. 3a shows the potential in thestructure in the direction perpendicular to the layers for varioussuccessive sections in the second dimension. In FIG. 3a, V_(SR) andV_(M) are the voltages applied to the superlattice 23 and to themodulator 30. The potential is shown for the equilibrium situation andwhen the device is biased.

Under these bias conditions, the superlattice oscillator 20 is excitedby a short (1 ps) optical pulse at 1.52 μm. The photocurrent induced isshown in FIG. 3b. It shows an oscillation at a frequency in the vicinityof 25 GHz in the superlattice 23. A photocurrent at the same frequencyis also induced in the modulator segment 30, which indicates that thereis a certain amount of crosstalk between the two paths. This effect isnot problematic when the oscillator 20 is locked by the optical signal:in which case the signal on the down path is centered on a highfrequency (25 GHz in the example modelled), and it is in base band onthe up path, and it is easy to effect filtering between the two paths.The same does not apply when the oscillator operates as a localoscillator and when the two paths are in base band.

A variant shown diagrammatically in FIG. 4 makes it possible to isolatethe two segments 20 and 30 electrically. It consists in replacingcertain n-type layers of the Bragg mirror 31 with non-doped or p-typelayers, e.g. one period (31b) of the five periods being a p-type periodflanked by two n-type periods (31a and 31c) on either side. Two separatecontacts 27 and 37 are then provided on the layers 25 and 31c, as shownin FIG. 4, where in FIG. 1, there is one common contact. These contacts27 and 37 are allocated respectively to the superlattice oscillator 20and to the modulator 30 forming the Fabry-Perot mirror. For thispurpose, the layer 31c extends beyond the superposed layers 32, 33 forreceiving the contact 37.

Second embodiment

In the second embodiment, the integrated vertical structure comprises ahollow-cavity Fabry-Perot mirror 30 and a superlattice oscillator 20.

FIG. 5 is a diagram showing this structure.

Many of the elements are common to both embodiments.

In particular, FIG. 5 shows a superlattice oscillator 20 comprising thefollowing deposited in succession on a semi-insulating substrate 10:

an n-type contact layer 21;

a graded layer 22;

a superlattice 23;

a graded layer 24; and

an n-type contact layer 25;

as well as two metal contacts 26, 27 deposited respectively on thecontact layer 21 and on the contact layer 25, as described above withreference to FIGS. 1 to 4.

Conversely, in the embodiment shown in FIG. 5, the modulator 30 isformed of a hollow Fabry-Perot cavity, i.e. the modulator 30 comprisestwo mirrors 31 and 33 separated by a layer of air 38.

The top mirror 33 is suspended by a spacer layer 32 above the bottommirror 31 which overlies the superlattice oscillator 20.

This type of embodiment can be obtained in two technological steps:etching the pattern in the top mirror 33 and chemically etching thelayer 32 flanked by being grown between the two mirrors 31 and 33 of thestructure 30. This layer 32 is designed to facilitate this treatment.

The top mirror 33 then behaves like a deformable membrane. The thicknessof the cavity 38 is variable and can be modulated under the effect of avoltage applied between the membrane 33 and the fixed bottom mirror 31by means of the electrodes 37, 36 deposited respectively on mirror layer31c and on mirror layer 33.

The resonance of the cavity 38 is thus modulable, and thus, at a fixedoperating wavelength, the reflectivity of the structure follows the samemodulation.

In a non-limiting example, the device of the present invention shown inFIG. 5 comprises the following succession of layers grown epitaxially onan InP substrate 10:

A first set 21-25 of layers forms the oscillator 20 that islight-sensitive in the vicinity of 1.5 μm:

1) An n-type (10¹⁹ cm⁻³) GaInAs contact layer 21 having a thickness ofabout 7,000 Å.

2) A superlattice 23 made of InGaAs/InGaAlAs or some other material thatis active in the vicinity of 1.5 μm, n-type doped at about 2×10¹⁶ cm⁻³.By way of example, the following pair of materials may be chosen:InGaAs/(InGaAs)₀.5 (InAlAs)₀.5, with wells of 60 Å, barriers of 20 Å,and a total thickness of 9,165 Å, for operation in the vicinity of 50GHz.

3) A contact layer 25 made of GaInAs of the n-type (10¹⁹ cm⁻³) having athickness of about 2,000 Å. The exact thicknesses of the layers 21, 23,and 25 may be chosen so that they are mλ/2 thick (where m is aninteger), Å being the reference wavelength for the design of themodulator. The layer 25 must absorb the residual 1.3 μm which has notbeen absorbed or reflected in the modulator 30, so as to isolate theoscillator 20 optically from the modulator 30. Its thickness may be1,845 Å.

A second set 31-33 of layers forms the 1.3 -μm modulator 30:

4) A Bragg mirror 31 reflective in a band centered on 1.3 μm, e.g. 26periods of an alternating succession of 2 layers as follows:

938 Å of (InGaAs)₀.62 (InAlAs)₀.38

1,005 Å of InAlAs

5) A layer 32 making selective chemical etching possible, e.g. made ofInP.

6) A p-doped Bragg mirror 33 (5×10¹⁸ cm⁻³) constituted by 2 layers asfollows:

1,005 Å of InAlAs

938 Å of (InGaAs)₀.62 (InAlAs)₀.38

In this embodiment too, over a small portion of its thickness, each ofthe contact layers 21 and 25 has a gradually changing composition 22, 24serving to eliminate sudden discontinuities in the conduction band.

The succession of the layers is the same as in the first embodiment upto layer 25. Layer 31 is a single 1.3 -μm Bragg mirror. Beyond it, layer32 must be removed by chemical etching. Its thickness is a multiple of0.65 μm. The layers 33 constitute a Bragg mirror.

FIG. 6 is a plan view of the structure of the modulator.

FIG. 7 shows a model of the reflectivity of the resulting device. Itstransmission is quite low between 1.5 μm and 1.54 μm, but varies verylittle, and therefore does not depend on the voltage applied to themodulator 30.

Naturally, the present invention is not limited to the above-describedparticular embodiments, but rather it extends to any variant lying withthe spirit of the invention.

The fiber 40 may apply the optical signals λ1, λ2 either via the top ofthe structure, in which case the oscillator 20 is deposited by epitaxialgrowth prior to the modulator 30, or via the bottom of the structure,through the substrate 10, in which case the modulator 30 is deposited byepitaxial growth prior to the oscillator 20. The latter configurationmakes it possible to reduce the surface area of the oscillator, whichmay be advantageous to reduce its capacitance and to reach the higherfrequencies in the millimetric range.

In a variant, the device of the present invention operates with theoscillator 20 being optically driven indirectly. In this variant, themodulator segment 30 is used alternately as a mirror modulator for theup path and as a photodetector for the down path. An electro-absorbantmodulator 30 delivers a photocurrent when it is absorbent. Thephotodetected signal then serves to drive the oscillator segment 20electrically.

With an alternating cycle, the duplexer is no longer used as a duplexer.The photosensitivity of the oscillator 20 and the transparency of themodulator 30 are not longer required, and it is not necessary to use twodifferent wavelengths. If, however, two difference wavelengths are used,it is advantageous to choose a wavelength for the photodetection that isslightly shorter than the working wavelength of the modulator 30 (e.g.less than 1.28 μm for the modulator 30 whose reflectivity is shown inFIG. 2). This variant makes it possible to improve the operatingflexibility and possibilities of the component.

The invention is applicable to any communications system architectureusing a both-way transposition interface between optical signals andelectrical signals, regardless of the approach envisaged depending onwhether or not the optical signal includes the radio carrier.

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We claim:
 1. A device for both-way transposition between optical signalsand electrical signals, for a communications system, said devicecomprising a vertically integrated component comprising an opticallydriven oscillator based on a negative differential conductancemechanism, for performing optical-to-millimetric conversion on the downpath, and an asymmetric Fabry-Perot cavity modulator for performing theelectrical-to-optical conversion function on the up path.
 2. A deviceaccording to claim 1, wherein the optically driven oscillator is chosenfrom the group comprising: Gunn diodes, IMPATT diodes, and superlatticediodes.
 3. A device according to claim 1, wherein the modulator servesas a mirror driven by a radio signal received by an antenna modulatingan optical carrier sent by an exchange.
 4. A device according to claim1, wherein the Fabry-Perot cavity modulator includes a plurality ofmirrors and at least one of the mirrors of the Fabry-Perot cavity isbased on semi-conductor layers.
 5. A device according to claim 1,wherein the modulator is transparent to the wavelength at which theoscillator is active.
 6. A device according to claim 1, including atleast one contact layer between the optically driven oscillator and theFabry-Perot cavity modulator, which contact layer is transparent to thewavelength to which the oscillator is sensitive, and organized so as notto impart electrical or optical coupling between the oscillator and themodulator.
 7. A device according to claim 1, further including a singleoptical fiber organized to convey to the component two wavelengths:namely a first wavelength to which the modulator is transparent and theoscillator is sensitive, and a second wavelength at which the modulatoroperates.
 8. A device according to claim 7, wherein the optical fiberconveys the optical signal to the top of the device, and the oscillatoris deposited by epitaxial growth prior to the modulator.
 9. A deviceaccording to claim 7, wherein the optical fiber conveys the opticalsignal to the bottom of the device, through a substrate, and theoscillator is deposited by epitaxial growth after the modulator.
 10. Adevice according to claim 1, wherein the modulator is organized toreflect a wavelength that is shorter than the wavelength to which theoscillator is sensitive.
 11. A device according to claim 1, wherein theoscillator comprises two contact layers flanking a superlattice.
 12. Adevice according to claim 11, wherein each of the two contact layers hasa gradually changing composition over a portion of its thicknessadjacent to the superlattice.
 13. A device according to claim 1, whereinthe oscillator comprises:an n-type GaInAs contact layer; a superlattice;and an n-type GaInAs contact layer.
 14. A device according to claim 13,wherein superlattice includes an the active layer made ofInGaAs/InGaAlAs.
 15. A device according to claim 13, wherein thesuperlattice includes an active layer made of InGaAs/(InGaAs)₀.5(InAlAs)₀.5.
 16. A device according to claim 1, wherein the modulator isa Fabry-Perot mirror modulator which comprises a back first mirror, anactive layer, and a front second mirror.
 17. A device according to claim16, wherein the active layer is a multiple quantum well layer.
 18. Adevice according to claim 16, wherein the back mirror of the modulatoris a Bragg mirror.
 19. A device according to claim 16, wherein the backmirror of the modulator is constituted by a periodically alternatingsuccession of materials of high refractive index and of low refractiveindex, and of quarter-wave thickness.
 20. A device according to claim16, wherein back mirror of the modulator is constituted by a stack ofsemi-conductor materials.
 21. A device according to claim 16, whereinthe first mirror of the modulator is formed of an alternating successionof layers made of (InGaAs)₀.62 (InAlAs)₀.38 and of InAlAs.
 22. A deviceaccording to claim 16, wherein front mirror of the modulator is a Braggmirror.
 23. A device according to claim 16, wherein the front mirror ofthe modulator is constituted by the interface between air and a layer ofsemiconductor material.
 24. A device according to claim 16, wherein thefront mirror of the modulator is constituted by a stack of insulatingdielectrics.
 25. A device according to claim 16, wherein the activelayer of the modulator is formed of (InGaAs)₀.73 (InAlAs)₀.27.
 26. Adevice according to claim 25, wherein a portion of the active layer isp-doped.
 27. A device according to claim 16, wherein the second mirroris constituted by an alternating succession of layers of InAlAs and of(InGaAs)₀.62 (InAlAs)₀.38.
 28. A device according to claim 1, whereinthe modulator is a hollow-cavity Fabry-Perot mirror.
 29. A deviceaccording to claim 28, wherein the modulator comprises a back firstmirror, a layer of air and a front second mirror.
 30. A device accordingto claim 28, wherein the etched layer of the modulator interposedbetween two mirrors is formed of InP.
 31. A device according to claim 1,including three electrodes of the oscillator, an electrode deposited onthe modulator, and an electrode that is common to the oscillator and tothe modulator.
 32. A device according to claim 1, including fourelectrodes: two electrodes deposited on respective contact layers of theoscillator, and two electrodes deposited on the modulator.
 33. A deviceaccording to claim 1, wherein the oscillator itself generates a radiocarrier and acts as a mixer for mixing its characteristic oscillation,imposed by the characteristics of the circuit in which it is inserted,with a data-carrying signal that arrives optically.
 34. A deviceaccording to claim 1, wherein the oscillator is an optically lockedoscillator, a radio carrier being conveyed by an optical distributionnetwork, and the frequency of the oscillator being servo-controlled andsynchronized continuously on the instantaneous value of the millimetricfrequency present on the optical signal.
 35. A device according to claim34, wherein the oscillator is locked directly, the optical signalservo-controlling the oscillator itself.
 36. A device according to claim34, wherein the oscillator is locked indirectly, said oscillator beinglocked by an electrical signal output by a distinct photodetector.
 37. Adevice according to claim 36, wherein the oscillator is locked by anelectrical signal output by a photodetector-forming modulator.
 38. Acommunications system comprising an optical fiber distribution networkbetween at least one exchange and a plurality of stations each of whichis equipped with a radio transmitter forming a radio access network atthe end of the optical fiber network, said communications systemincluding a both-way transposition device according to claim 1 in eachof the stations.